1. Field of the Invention
The present invention relates to turbo machines and more particularly doubleflow aircraft turbojet engines (i.e. turbo-engines operating with two concentric annular jets) equipped with bleed (i.e. discharge) valves.
2. Discussion of the Background
With reference to FIG. 1, such turbojet engines conventionally have upstream in the primary flow, i.e. in the internal channel 2, a low pressure axial compressor 3 supplying compressed air to a combustion chamber 8 in which the air is mixed with pressurized fuel. This mixture is burned in order to provide energy downstream of the chamber 8 to an axial turbine, which drives the high pressure compressor 4 and wherein the exiting gases supply the thrust necessary for propulsion of the aircraft. Such turbojet engines have a completely axial configuration. As a result of their frontal intake, they not only absorb the air necessary for their operation but, as a function of the climatological conditions, also sand and water, which are prejudicial to the satisfactory operation of the turbojet engine.
This is in particular the case when the aircraft is confronted with a thunderstorm or passes through a dense, large volume cloud, such as a cumulus or cumulonimbus cloud. Large amounts of water in the form of rain or hail then enter the compressor. If the engine is under full power, the water is immediately evaporated. If it penetrates to the combustion chamber, it is in the form of sufficiently hot vapor and is atomized. It does not lead to the flame-out of the combustion chamber, which is then supplied with a large fuel flow. However, this is not the case when the aircraft is descending, e.g. during an approach phase prior to landing. In this case the turbojet engine is under slow-down conditions, which leads to a low compression ratio and fuel flow. If a significant flow of water or ice fragments then reaches the combustion chamber, this can lead to the extinguishing of the burners. Wet igniter plugs are unable to function when engine flame-out has occurred. If the pilot is unable to leave the critical rain area, the engine or engines can then be completely stopped, with the ensuing risks. This is the common flame-out mode of all the engines.
In order to avoid such problems, which can prove catastrophic, it is known to eliminate the water which has entered the combustion chamber in order to prevent the extinguishing of the burners. For this purpose use is made of bleed valves 1, which open the internal channel 2 onto the external channel 6, said valves being placed between the upstream compressor 3 and the downstream compressor 4. As is shown by the arrow 5, part of the flow returned into the upstream compressor 3 is deflected towards the external channel 6, so that the water or ice liable to seriously prejudice the operation of the turbojet engine is deflected.
These bleed valves 1 are originally provided for preventing the "pumping" phenomenon of the low pressure compressor 3, when it is necessary to pass to it a flow rate exceeding that which could be accepted by the downstream high pressure compressor 4.
With reference to FIG. 2, normal operation of the discharge valves follows the curve shown in solid line form therein. On the abscissa is plotted the operating conditions of the engine and on the ordinate the opening of the valves. As is apparent from the thickline curve, on starting the engine the valves are in the quasi-open position. The valve closure law follows the curve, which decreases in a relatively regular manner and then levels out in front of point C. The curve touches the valve closing line at point D, which corresponds to full power operation.
This operation is realized for the bleed valves by control means and preprogrammed hydromechanical regulating means in order to control the opening and closing of the valves in accordance with said curve.